Zigah Prosper

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Zigah
First Name
Prosper
ORCID
0000-0001-7857-9751

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  • Preprint
    Constraining the sources and cycling of dissolved organic carbon in a large oligotrophic lake using radiocarbon analyses
    ( 2017-03) Zigah, Prosper ; Minor, Elizabeth C. ; McNichol, Ann P. ; Xu, Li ; Werne, Josef P.
    We measured the concentrations and isotopic compositions of solid phase extracted (SPE) dissolved organic carbon (DOC) and high molecular weight (HMW) DOC and their constituent organic components in order to better constrain the sources and cycling of DOC in a large oligotrophic lacustrine system (Lake Superior, North America). SPE DOC constituted a significant proportion (41-71 %) of the lake DOC relative to HMW DOC (10-13%). Substantial contribution of 14C-depleted components to both SPE DOC (Δ14C = 25 to 43‰) and HMW DOC (Δ14C = 22 to 32‰) was evident during spring mixing, and depressed their radiocarbon values relative to the lake dissolved inorganic carbon (DIC; Δ14C ~ 59‰). There was preferential removal of 14C-depleted (older) and thermally recalcitrant components from HMW DOC and SPE DOC in the summer. Contemporary photoautotrophic addition to HMW DOC was observed during summer stratification in contrast to SPE DOC, which decreased in concentration during stratification. Serial thermal oxidation radiocarbon analysis revealed a diversity of sources (both contemporary and older) within the SPE DOC, and also showed distinct components within the HMW DOC. The thermally labile components of HMW DOC were 14C-enriched and are attributed to heteropolysaccharides (HPS), peptides/amide and amino sugars (AMS) relative to the thermally recalcitrant components reflecting the presence of older material, perhaps carboxylic-rich alicyclic molecules (CRAM). The solvent extractable lipid-like fraction of HMW DOC was very 14C-depleted (as old as 1270-2320 14C years) relative to the carbohydrate-like and protein-like substances isolated by acid hydrolysis of HMW DOC. Our data constrain relative influences of contemporary DOC and old DOC, and DOC cycling in a modern freshwater ecosystem.
  • Preprint
    Assessing the blank carbon contribution, isotope mass balance, and kinetic isotope fractionation of the Ramped Pyrolysis/Oxidation instrument at NOSAMS
    ( 2017-03) Hemingway, Jordon D. ; Galy, Valier ; Gagnon, Alan R. ; Grant, Katherine E. ; Rosengard, Sarah Z. ; Soulet, Guillaume ; Zigah, Prosper ; McNichol, Ann P.
    We estimate the blank carbon mass over the course of a typical Ramped PyrOx (RPO) analysis (150 to 1000 °C; 5 °C×min-1) to be (3.7 ± 0.6) μg C with an Fm value of 0.555 ± 0.042 and a δ13C value of (-29.0 ± 0.1) ‰ VPDB. Additionally, we provide equations for RPO Fm and δ13C blank corrections, including associated error propagation. By comparing RPO mass-weighted mean and independently measured bulk δ13C values for a compilation of environmental samples and standard reference materials (SRMs), we observe a small yet consistent 13C depletion within the RPO instrument (mean – bulk: μ = -0.8 ‰; ±1σ = 0.9 ‰; n = 66). In contrast, because they are fractionation-corrected by definition, mass-weighted mean Fm values accurately match bulk measurements (mean – bulk: μ = 0.005; ±1σ = 0.014; n = 36). Lastly, we show there exists no significant intra-sample δ13C variability across carbonate SRM peaks, indicating minimal mass-dependent kinetic isotope fractionation during RPO analysis. These data are best explained by a difference in activation energy between 13C- and 12C-containing compounds (13–12ΔE) of 0.3 to 1.8 J×mol-1, indicating that blank and mass-balance corrected RPO δ13C values accurately retain carbon source isotope signals to within 1 to 2‰.
  • Article
    Allochthonous sources and dynamic cycling of ocean dissolved organic carbon revealed by carbon isotopes
    (John Wiley & Sons, 2017-03-07) Zigah, Prosper ; McNichol, Ann P. ; Xu, Li ; Johnson, Carl G. ; Santinelli, Chiara ; Karl, David M. ; Repeta, Daniel J.
    We present concentration and isotopic profiles of total, size, and polarity fractionated dissolved organic carbon (DOC) from Station ALOHA (A Long-term Oligotrophic Habitat Assessment), an oligotrophic site in the North Pacific Ocean. The data show that, between the surface and 3500 m, low molecular weight (LMW) hydrophilic DOC, LMW hydrophobic DOC, and high molecular weight (HMW) DOC constitute 22–33%, 45–52%, and 23–35% of DOC, respectively. LMW hydrophilic DOC is more isotopically depleted (δ13C of −23.9‰ to −31.5‰ and Δ14C of −304‰ to −795‰; mean age of 2850 to 15000 years) than the LMW hydrophobic DOC (δ13C of −22‰ to −23‰ and Δ14C of −270‰ to −568‰; 2470 to 6680 years) and HMW DOC (δ13C of ~−21‰ and Δ14C of −24‰ to −294‰; 135–2700 years). Our analyses suggest that a large fraction of DOC may be derived from allochthonous sources such as terrestrial and hydrothermal DOC and cycle on much longer time scales of >10000 years or enter the ocean as preaged carbon.